This anatomical-physiological investigation is designed to systematically evaluate the effects of loud sound on auditory system function. Initial studies of the first year are designed to permit the collection of quantitative measures of a number of neural properties common to single cochlear nucleus cells in the awake monkey before and following the introduction of moderately-intense stimuli that are systematically related to unit characteristic frequency. Particular attention will be given to the anatomical location of these cells within the three major subdivisions of the cochlear nucleus so that we can begin to more precisely relate the physiological effects of exposure stimuli to the known morphological properties of neurons in these discrete regions. Additionally, physiological findings including preexposure sensitivity and the average magnitude of threshold shift over designated frequency ranges will be related to basic anatomical characteristics (i.e., basilar membrane length, hair-cell density) of the cochlear duct. Further, an evaluation of the pathological effects of repeated episodes of sound exposure on end-organ elements using plastic-embedding histological techniques will be performed. During YEARS 2 and 3, a series of analytic studies in the acute, anesthetized monkey are proposed that will permit, by use of anatomical labeling techniques, a more precise determination of the relation of underlying cellular morphology to the pathophysiological response.